ThumbsPlus uses the XMP format, which stores all metadata an an XML tree, and Adobe's XMP library, to store and maintain metadata. ThumbsPlus has its own internal list of "supported" XMP fields, but can display metadata not on that list.
ThumbsPlus allows you to modify or add many fields, but for compatibility with other applications, only those on its supported list, and only those which make sense to edit. For example, it doesn't allow you to edit the Exposure Time, since that is static information about how the image was created.
The Metadata panel shows the metadata for the current file. When you have selected a single file, you may edit the supported metadata in that file directly within the panel. To edit the metadata in multiple files, use the Batch Edit Metadata function on the Image menu.
Metadata in the ThumbsPlus database
ThumbsPlus automatically imports metadata for several items so they may be used to quickly search for images based it. These database items are handled automatically:
Date Taken
ThumbsPlus loads the Date Take in the database from the XMP/EXIF field Original Date Time.
Rating
ThumbsPlus will load Rating from the XMP rating field (xmp:rating) or the Microsoft rating field (MicrosoftPhoto:Rating). The Microsoft rating is converted from its 0-100 range to the standard 0-5.
Keywords
Database keywords are automatically created from XMP keywords, based on the XMP Keywords setting in Keyword Preferences. (For historical reasons, these are stored in the XMP bag named dc:subject. ThumbsPlus and most other software always refer to them as keywords.)
Annotations
Subject to the option in the Metadata Preferences, ThumbsPlus copies the XMP Caption (also known as Description and Comment in some software) to the database Annotation field.
User Fields
You can also create User Fields for any metadata item, and if it has the exact same name as an XMP field, ThumbsPlus will automatically populate the field from the metadata item. All metadata fields in the database should be stored as Text, rather than numeric or date/time. You may disable this behavior in Metadata Preferences.